JP2004327992A - Semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce ringing without substantially shortening the lifetime of a laser and without increasing power consumption. <P>SOLUTION: The laser, a p-spacer (67), an n-spacer (66), and an active region (63) are disclosed. The p-spacer has a p-doped region (57) and an undoped region (58), and the n-spacer has an n-doped region (69) and an undoped region (68). The active region is located between the undoped region of the p-spacer and the undoped region of the n-spacer. The active region generates light of wavelength λ through the recombination of holes and electrons. The undoped region of the p-spacer has a thickness that is different from that of the undoped region of the n-spacer, in one embodiment, the former has a thickness greater than the latter. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a semiconductor laser.

  Communication system bandwidth and distance requirements are ever-increasing, and data is currently transmitted over optical fibers. Conventional telecommunications and data networks, such as the Internet, are increasingly using optical fibers for both short and long distance transmissions. Optical communication channels provide data rates in excess of 10 Gbit / sec relatively inexpensively. Data transmitted downstream on such a channel is typically generated in the form of an electrical signal, which is converted to an optical signal by directly modulating a laser at one end of an optical fiber.

  Vertical cavity surface emitting lasers (VCSELs) are becoming commercially important as transmitters in high bit rate (> 1 Gb / s) optical communication links. In such optical communication links, the VCSEL is modulated in a large signal form with a digital bit pattern, so the characteristics of the VCSEL have a significant effect on the fidelity of converting an electrical bit pattern to an optical pattern. Consider a VCSEL that is modulated by an electrical signal that includes a step function that switches from 0 to some predetermined value in a very short period of time. Ideally, the output of the laser is an optical signal that switches from zero intensity to some predetermined intensity value within a similar time period. Unfortunately, at very high switching speeds, the output of the laser exhibits a slowly decreasing ringing pattern superimposed on the step function. The oscillation frequency of the ringing pattern is related to the relaxation frequency of the laser, which is determined by the electro-optical resonance or relaxation frequency of the laser cavity. Damping inside the cavity determines the rate at which ringing disappears.

  A similar ringing phenomenon occurs when the laser is turned off by the transition of the modulated signal from 1 to 0. At high modulation frequencies, the ringing of light intensity can cause data errors at the receiver. Therefore, reducing this ringing effect is an important consideration in laser design and modulation protocols.

  One conventional approach to reducing the ringing effect and thereby increasing the modulation frequency is that both the damping factor and the relaxation frequency increase with increasing power level or bias current. Based on observation. The purpose of such an approach is to move the relaxation frequency to a value sufficiently higher than the modulation frequency to dampen the vibration as soon as possible. If the relaxation frequency is sufficiently above the modulation frequency, the laser can follow the drive signal without significant ringing. With a suitable low-pass filter, residual low-level ringing can also be removed. Similarly, as the damping factor increases, the amplitude and duration of the ringing is reduced. Therefore, the effect of ringing can be reduced by increasing the power flowing through the VCSEL.

  Unfortunately, the operating life of a VCSEL is related to the power level at which it is driven. For example, the failure rate of a VCSEL increases with the cube of the current density in the device. That is, if the current density doubles, the failure rate increases eight times. Therefore, the above-described method of reducing the ringing effect substantially shortens the life of the VCSEL. Higher bias currents also increase the power consumption of data communication transceivers.

The present invention includes a laser, a p-spacer, an n-spacer, and an active region. The p-spacer has a p-doped region and an undoped region, and the n-spacer includes an n-doped region and an undoped region. The active region is located between the undoped region of the p-spacer and the undoped region of the n-spacer. The active region recombines holes and electrons to generate light of wavelength λ. The undoped region of the p-spacer has a different thickness than the undoped region of the n-spacer. In one embodiment, the thickness of the p-spacer is greater than the thickness of the n-spacer. If the laser is a VCSEL, the p-spacer and the n-spacer form an optical cavity having a thickness L _c = (n + 1) λ / 2, where n is an integer greater than two. In another embodiment, the cavity has a standing electromagnetic wave and the active layer is located at a maximum of the electromagnetic standing wave.

  How the present invention provides advantages can be more readily understood with reference to FIG. FIG. 1 shows layers in a typical VCSEL 10 according to one embodiment of the present invention. To simplify the drawings, the thicknesses of the various layers are not drawn to scale. The VCSEL 10 is configured on a substrate 11, and in this embodiment, the substrate 11 includes any contact layers required to supply power below the active region. The VCSEL 10 includes a lower mirror 12, an optical cavity 13, and an upper mirror 14. Power is supplied via the upper contact layer 20. The mirror is a conventional DBR mirror and will not be described in detail herein. For the purposes of this description, it is sufficient to note that the mirror is constructed by alternately growing layers of material such that adjacent layers have different refractive indices. Typically, the lower mirror comprises an n-type material and the upper mirror comprises a p-type material. However, other designs can be used without departing from the teachings of the present invention.

  The laser cavity includes an active region 15. The active area is shown in detail in FIG. 2, which is an enlarged view of the active area shown in FIG. The active region includes a quantum well layer 25 separated by a barrier layer 26. The spacer layers, indicated at 27 and 28, are used to set the length of the cavity to the desired length for the wavelength of light generated. Also, the spacer layer is used to position the active region at the peak of the standing wave in the cavity formed between the upper and lower mirrors. The layers of the present invention and the order of the layers are similar to those used in prior art VCSELs. However, as will be described in more detail below, in the present invention, the thickness of the layers and the starting and ending points of the doping are varied. In prior art VCSELs, the spacer layer typically has a thickness set such that the combined thickness of the two spacer layers and the active region is equal to λ. Where λ is the wavelength of light emitted by the material in the optical cavity. Since the thickness of the active region is small compared to λ, the spacer layer is about λ / 2 for prior art devices. In a VCSEL, the active region is located at the peak of the light standing wave in the cavity.

  Referring to FIG. 3, there is shown the light output of a typical prior art VCSEL when driven with a step function increasing from zero to a fixed value at t = 0. As can be seen, the output of the laser oscillates (ringing) for approximately the period shown in region 31 of the graph. If the amplitude of the ringing causes the signal to drop below half the light level corresponding to a logical one, a receiver at the other end of the fiber can mistake such ringing for a one-to-zero transition. There is.

  The present invention states that by introducing a delay in the time required for holes or electrons to reach the active region, the amplitude of the ringing can be reduced without changing the power level driving the device. Based on observation. Although the mirrors in VCSELs are usually doped, the spacer regions in prior art VCSELs are also lightly doped, and doping of the spacer regions ends at a distance of several tens of nm from the active region. When a potential is applied across the mirror, holes are injected into the spacer adjacent to the p-doped mirror and electrons are injected into the spacer adjacent to the n-doped mirror. The holes and electrons move to the active region where they combine to generate light. In prior art VCSELs, doping terminates near the active region (ie, tens of nm from the active region), minimizing the time for carriers to reach the quantum well.

In operation of the laser, a fixed DC current bias is applied to the laser, and a modulated AC bias is superimposed. The change in AC current changes the concentration of electrons and holes in the spacer layer, which takes some time to move to the active region for the holes and electrons to eventually recombine. This time delay affects the modulation response of the laser. Referring now to FIG. 4, there is shown a doping pattern in the region of a laser cavity in a typical VCSEL. Mirrors designated 51 and 52 define the cavity. Mirror 51 is doped with a p-type material and mirror 52 is doped with an n-type material. Distance L _c between the mirrors is typically set to 1 [lambda, where λ is the wavelength of light VCSEL generates. However, embodiments where L _c is equal to (n + 1) λ / 2 and n is an integer greater than 1 are also known in the art. Generally, the p-doped and n-doped regions terminate at the spacer region, as indicated by 54 and 55. The holes generated in the p-doped region diffuse through a length L _h undoped region in the spacer adjacent to the p-doped mirror before being captured in the quantum well layer indicated by 53. There must be. Similarly, electrons leaving the n-doped region of the spacer 55 also must diffuse through the non-doped region of the length L _e of the spacer adjacent to the n-doped mirror. The undoped region has a doping concentration of less than 5 × 10 ¹⁷ cm ⁻³ . In VCSEL of the prior art, in order to reduce the diffusion time, _{L h} and _{L e} is 25nm less. In a typical III-V semiconductor, the electron diffusion time is one fifth of the hole diffusion time.

The present invention is based on the observation that the extinction coefficient is related to the time required for electrons and holes to diffuse through these undoped regions. It is shown that as this delay increases, so does the attenuation coefficient. In particular, it can be shown that the delay through the spacer region acts similarly to the parasitic RC time constant in the electric circuit. The delay is given by the following equation (1).

In formula (1), L _h is the distance required for the holes to reach the quantum well diffused, L _e is the distance required for the electrons reach the quantum well diffused. _Dh and _De are the diffusion coefficients of holes and electrons. D _h but is approximately ^4cm 2 / sec, _{D e} is approximately 20 cm ² / sec. Therefore, even if the distance by which holes diffuse in the spacer layer on the hole side of the active layer is slightly increased, the delay, that is, the capture time is greatly increased. Therefore, the preferred embodiment of the present invention is realized by increasing this distance.

Referring now to FIG. 5, there is shown a doping pattern in the region of a laser cavity in a VCSEL according to the present invention. The cavity is defined by mirrors indicated by 61 and 62. Mirror 61 is doped with a p-type material and mirror 62 is doped with an n-type material. The distance _{L c} between the mirrors is set to (n + 1) λ / 2 , λ is the wavelength of light VCSEL is generated within the cavity material. In a preferred embodiment of the present invention, n> 2. The p-type doping ends at a location 64 in the p-type spacer 67, which has a doped region 57 and an undoped region 58. The undoped region provides a distance L _h for holes to diffuse before reaching the quantum well layer indicated by 63. Similarly, n-type spacer 66 includes an undoped region 68 and a doped region 69. n-type doping ends at position 65 of the n-type spacer, electrons, the distance L _e to diffuse before entering the quantum well layer. In one preferred embodiment of the present invention, the thickness of the p spacer layer is selected to be greater than λ / 2 and the thickness of the n spacer layer is selected to be less than λ / 2. The thickness of the undoped region of the p-spacer layer (L _h ) is selected to be greater than 20 nm, and the thickness of the undoped region in the n-spacer layer (L _e ) is selected to be less than 40 nm.

The exact value of L _h and L _e is determined by the desired degree of relaxation frequency and attenuation required. The relaxation frequency must be higher than the modulation frequency. The relaxation frequency is set by the current flowing through the device. Thus, once the modulation frequency is set, the appropriate relaxation frequency is determined by conventional means. For the purpose of this description, it is sufficient to note that the relaxation frequency is determined by the differential gain of the active region, the magnitude of the optical mode, and the bias current. It also defines the current density that must flow through the device. As described above, it is preferable to lower the current density in order to improve the life of the device. Thus, the target value of the relaxation frequency should be set as low as possible to provide the lowest bias and modulation current consistent with the demodulation circuit that must demodulate the modulated signal after transmission. .

Once the current density is set, L _h is varied and _Le is kept constant to find a value that provides optimal damping at this current density. Typically, the amount of attenuation required depends heavily on the material properties of the quantum well and the optical loss in the laser cavity, so the determination of the value is made experimentally.

  The area between the upper and lower mirrors forms a cavity with an electromagnetic standing wave when current is flowing through the laser. Usually, the active layer of the laser is arranged such that the quantum well layer is located at the maximum of this standing wave. In a conventional VCSEL, there is one such maximum at the center of the cavity and one such maximum at each of the mirror / spacer boundaries. In a VCSEL according to the invention, there can be some additional maximum in the cavity. Expanding only the p-spacer, these additional maxima will be on the p-spacer side of the quantum well. However, preferably, the quantum well is located at the maximum of the standing wave closest to the n-type mirror.

  The above embodiments of the present invention use enlarged undoped regions in the p-type spacer. However, as noted above, the purpose of increasing the delay between injecting carriers into the undoped spacer region and capturing the carriers by the quantum well is to reduce the thickness of the undoped region of the n-type spacer. It can also be achieved by increasing. However, typically, to obtain a given increase in delay time, the increase in thickness required for n-type spacers is less than the increase in thickness required to delay using p-type spacers. Since it is five times, the above method is preferable.

The method of the invention is also applicable to edge-emitting lasers. The use of the present invention in an edge emitting laser can be more easily understood with reference to FIG. FIG. 6 shows a basic structure of the edge emitting laser 70. Since edge emitting lasers are well known in the art, the detailed structure of laser 70 will not be described herein. For the purpose of this description, it is sufficient to note that laser 70 is a pin diode structure deposited on substrate 71. Typically, several layers are deposited on the substrate to form an n region 72, a spacer region 75, an active region 74 having one or more quantum well layers, a second spacer region 76, a p region 73. I do. The multilayer structure is etched to form a ridge 79 or buried structure that acts as a waveguide. Resonance frequency is set at a distance L _c between the end and the end of the ridge structure is cleaved to form the mirror.

  The distance between the edge of the p-region and the edge of the n-region is determined by the requirement that the waveguide must support only one optical mode in the vertical direction. It is determined. Since the thickness of the two spacer regions and the active region is less than one wavelength and the p-spacer is thicker than the n-spacer, the active region is not always located exactly at the maximum of the electric field in the cavity. However, the active region can be located sufficiently close to the point of maximum electric field that the laser can operate satisfactorily.

In an edge-emitting laser according to the present invention, the distance between the active region and the end of the p-doping is adjusted to introduce attenuation in a manner similar to that described above. Referring to FIG. 7, the doping pattern in the region of the laser cavity in the edge emitting laser 70 is shown. The vertical distance through a spacer region and the active region indicated by L _v. The doping of p region 73 can be continued up to point 84 in spacer region 76. Similarly, the doping of n region 72 can be continued up to point 85 in spacer region 75.

  The above embodiments of the present invention use a conventional pin laser diode structure. However, the invention can also be implemented in laser structures using reverse-biased tunnel junctions. In this type of laser, both the upper and lower contact layers are n-type. Lasers typically have an active layer configured on a lower n-type contact layer. An np reverse-biased tunnel diode junction is deposited over the p-type spacer in the active region, and the p-layer of the junction contacts the p-doped region of the p-spacer. The upper mirror and the upper contact are composed of n-type layers. This type of structure reduces current spreading and resistance problems associated with p-type contact layers used in conventional lasers. A light emitting device of this type is taught in US patent application Ser. No. 09 / 586,406, filed Jun. 2, 2000, which is incorporated herein by reference.

  Various modifications of the invention will be apparent to those skilled in the art from the foregoing description and drawings. Accordingly, the invention is limited only by the claims.

FIG. 2 illustrates the layers of a typical VCSEL 10 according to one embodiment of the present invention. FIG. 2 is an enlarged view of the active region shown in FIG. 1. FIG. 4 shows the light output of a typical prior art VCSEL driven by a step function increasing from 0 to a fixed value at t = 0. FIG. 4 shows a doping pattern in the region of a laser cavity in a typical VCSEL. FIG. 4 illustrates a doping pattern in the region of a laser cavity in a VCSEL, according to another embodiment of the present invention. FIG. 4 is a diagram showing a basic structure of an edge-emitting laser according to another embodiment of the present invention. FIG. 4 is a diagram illustrating a doping pattern in a region of a laser cavity of an edge-emitting laser according to another embodiment of the present invention.

Explanation of reference numerals

10 VCSEL
Reference Signs List 12 lower mirror 13 optical cavity 14 upper mirror 15 active region 25 quantum well layer 27 spacer layer 28 spacer layer 67 p spacer 66 n spacer 57 p doped region 58 undoped region 69 n doped region 68 undoped region 63 active region

Claims (6)

a p-spacer having a p-doped region and an undoped region;
an n spacer having an n-doped region and an undoped region;
An active region between the undoped region of the p-spacer and the undoped region of the n-spacer, wherein the active region generates light of wavelength λ by recombination of holes and electrons,
The undoped region of the p-spacer has a different thickness than the undoped region of the n-spacer;
laser. The laser of claim 1, wherein the p-spacer has a greater thickness than the n-spacer. The laser of claim 1, wherein the p spacer and the n spacer form an optical cavity having a thickness L _c = (n + 1) λ / 2, where n is an integer greater than two. The laser of claim 1, wherein the undoped region of the p-spacer has a thickness greater than 20nm. The laser of claim 1, wherein the undoped region of the n-spacer has a thickness less than 40 nm. 4. The laser according to claim 3, wherein the cavity has an electromagnetic standing wave, and the active layer is located at a maximum of the electromagnetic standing wave.

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